The Sinsk Biota: Post-Crisis Early Cambrian Lagerstätte on the Carbonate Siberian Platform

The early Cambrian (late Age 3/early Age 4) Sinsk Biota of the Siberian Platform represents an unusual Cambrian Lagerstätte. This fossil accumulation was formed on a carbonate platform and fossils were buried in calcareous organic-rich mudstone rather than in argillaceous lithologies, which were typical of the majority of Cambrian Lagerstätten. Besides, the Sinsk Biota consisted of disaster species that survived and proliferated shortly after a severe mass extinction coined the Sinsk Event (about 513 million years ago). The principal groups forming this biota were fleshy algae, stem spiculate siliceous sponges, linguliformean brachiopods, stem cycloneuralians, bradoriids and trilobites. All these groups were abundant (multiple individuals) and diverse (numerous species). Presumable cnidarians (Cambrorhytium), tentative deuterostomes (Eldonia) and trilobitomorphs (Phytophilaspis) were numerous but monospecific. The fossil preservation varies from carbonaceous compressions to pyritized and phosphatized tree-dimensional soft tissues depending on taxonomic affinities of fossils. Low oxic conditions, a fine-grained substrate and, probably, microbial activity facilitated the preservation of soft tissues, scleritomes and weakly biomineralized skeletons. Noteworthy, that pyritization mostly developed on organic-rich skeletonized fossils but not on pure organic remains, which could be indicative for an increased iron content of pyritized organisms. The Sinsk biota inhabited relatively deep low oxic marginal sea of the Siberian palaeocontinent and showed different adaptations to this stressful environment allowing metazoans to physiologically buffer in low oxygen conditions. Nonetheless, some metazoans got a relatively vacant space to diversify. Siliceous spiculate sponges were among them and reached enormous sizes (up to 0.5 m in diameter), a high diversity (about two dozen species) and disparity expressed in a high variety of spicule types including the earliest distinct microscleres. Ecologically, the Sinsk Biota formed a relatively simple palaeocommunity restricted to sessile/semisessile filtrators and motile, both infaunal and epibenthic, unspecialized predators. The research is supported by the Russian Science Foundation, grant 25-27-00582.